Usually, it needs several spin coating steps for fabricating a multilevel integrated circuit in a semiconductor manufacturing process. The theory of a spin coating process is to spray a liquid material on the surface of a spinning wafer and allow the liquid material to spread over the wafer evenly by the centrifugal force. After drying, a layer with an uniform thickness is formed on the surface of the wafer. Almost all the photoresist layers are formed by this kind of spin coating process now. The result of the sequential photolithographic process will be influence by the uniformity of the photoresist layer. If the thickness of the photoresist layer is not uniform, the quality of the transferred pattern is poor. This can cause a short circuit after several photolithographic steps. A photoresist layer without any defect can be made in the spin coating step; therefore, the spin coating step is very important in the semiconductor manufacturing process.
The thickness of the photoresist layer is related to the resolution of the photolithography. The photoresist layer becomes thinner, the resolution becomes higher. However, in another aspect of view, a thick photoresist layer has a better etch resistance and few contaminants. Therefore, the thickness of the photoresist layer should be controlled in a proper range. There are many parameters determining the thickness of a photoresist layer and the most important one is the spinning rate of the spin coater. The equation used for calculating the thickness of a photoresist layer is: ##EQU2## where t is the thickness of the photoresist layer, k is a factor varied along with the viscosity of the liquid material, S is the solid content in the photoresist layer, and w is the spinning rate of the coater.
If several wafers are coated by the same liquid material, k becomes a constant but S would be varied according to the different thickness of the photoresist layers. Although the range of different thickness are not very large, the required quality of the semiconductor device is very high now and it is impossible to ignore this variation of S.
Now, k and S are still viewed as constants when calculating the thickness of a photoresist layer; therefore, ##EQU3## By a specific spinning rate of the spin coater and its corresponding thickness of the photoresist layer, we can estimate another thickness of a photoresist layer formed by another specific spinning rate or estimate another spinning rate used to form another specific thickness of photoresist layer. The equation for calculating the thickness of the photoresist layer or the spinning rate of the coater is: ##EQU4## Because S is viewed as a constant, the error of the calculated thickness or the spinning rate is small only when the thickness of two photoresist layers are very similar. If the difference between each thickness of two photoresist layers become large, the error of calculating an unknown thickness of the photoresist layer by equation (3) can not be ignored. Therefore, it is tried by the applicant to deal with the problem encountered with the prior art.